1. Field of the Invention
The present invention relates to accelerator apparatus provided in diaphragm carburetors for supplying fuel to compact, general purpose engines.
2. Description of Related Art
It is known that an accelerator apparatus having an accelerator pump mechanically coupled to a throttle valve may be provided as a means for increasing a rate of fuel flow delivered to an intake passage of a carburetor in response to an increased amount of air intake, during accelerated operation of an engine.
In known diaphragm carburetors, as depicted in conceptual diagrams in FIGS. 2A, 2B, and 2C, in which fuel delivered from a fuel tank by a fuel pump is held in a fixed fuel chamber at a fixed pressure by utilizing a diaphragm and an atmospheric pressure. Such fuel then is delivered to an intake passage from the fixed fuel chamber and supplied to an engine.
Referring to the structure shown in FIG. 2A, which is described, for example, in Japanese Unexamined Patent Publication No. 60-195365, the disclosure of which is incorporated herein by reference; a main jet 54 defining a maximum rate of fuel flow and a regulating needle valve 55 regulating a rate of fuel flow are disposed in a main fuel passage 53 extending from a fixed fuel chamber 51 to a main nozzle port 52. Further, a pump chamber 58 of an accelerator pump 57 is connected to main fuel passage 53 downstream of regulating needle valve 55 by a single accelerated fuel passage 59. Further, referring to the structure shown in FIG. 2B, which is described, for example, in Japanese Unexamined Patent Publication No. 10-238411, the disclosure of which is incorporated herein by reference; a check valve 56, which prevents air from flowing into main jet 54 and fixed fuel chamber 51, and regulating needle valve 55 are disposed in main fuel passage 53. Accelerated fuel passage 59 is connected to main fuel passage 53 between main jet 54 and check valve 56. Further, referring to the structure shown in FIG. 2C, which is described, for example, in Japanese Unexamined Utility Model Publication No. 6-67842 and Japanese Unexamined Patent Publication No. 10-213013, the disclosures of which are incorporated herein by reference; check valve 56 and regulating needle valve 55 are disposed in main fuel passage 53. Accelerated fuel passage 59 is connected to main fuel passage 53 between check valve 56 and regulating needle valve 55.
Accelerator pump 57 and accelerated fuel passage 59 define an accelerator apparatus. Regardless, whether the throttle valve is a butterfly throttle valve or a rotary throttle valve, as described in each of the Japanese references mentioned above, accelerator pump 57 discharges fuel in pump chamber 58 during acceleration and is coupled mechanically to the throttle valve so as to deliver fuel to main nozzle port 52 from accelerated fuel passage 59 and main fuel passage 53, and accelerator pump 57 draws in fuel in fixed fuel chamber 51 to pump chamber 58 from main fuel passage 53 and accelerated fuel passage 59 during deceleration.
In the accelerator apparatus described above, connecting accelerated fuel passage 59 to main fuel passage 53 has been attempted to improve the carburetor's acceleration response. Nevertheless, when introducing fuel into a newly manufactured carburetor or after the carburetor has not been in use for a long period of time, it frequently occurs that air remains in pump chamber 58. Further, if fuel vapor is generated in pump chamber 58 due to engine heat which remains after the engine is shut down, such fuel vapor may accumulate within pump chamber 58 due to a positional attitude of accelerator pump 57 and a configuration of accelerated fuel passage 59. Further, air and fuel vapor are discharged little by little during engine operation so as to enter into main fuel passage 53, and disrupt the rate of fuel flow set by main jet 54 and regulating needle valve 55. In particular, in the structure shown in FIG. 2A, discharged air and fuel vapor entering main fuel passage 53 adversely affect the regulation of the rate of fuel flow of regulating needle valve 55. Similarly, in the structures shown in FIGS. 2B and 2C, discharged air and fuel vapor entering main fuel passage 53 dilutes an air-fuel mixture during discharging. Further, fuel vapor exerts a pumping effect, pushing out fuel within pump chamber 58 due to vibration of the engine during engine operation or a negative pressure pulsation of a venturi portion to which main nozzle port 52 is open. As a result, and fuel may be delivered to the intake passage irregularly so as to make an air-fuel ratio unstable.
As a result of these phenomena, undesirable results may occur. For example, engine operation may become unstable, and the content of harmful materials may be increased in the exhaust gas.